If an unsupported airplane doesn't use standard FSX variables to control its doors, either you know how the variable is checked (so you can add a custom XML expression to check it), or you should set that door to be Ignored, so GSX won't ask for it.
The Maddog surely uses custom variables for service doors, but Leonardo was the only 3rd party airplane developer that never replied to any of our enquiries about it (which is funny, since it's the only other developer I could write to in my language...), so you'll have to rely on the Ignore option.
